Comparing Init Accessors and Readonly Fields in C#

In C#, managing immutable data is crucial for building reliable and thread-safe applications. Both init accessors and readonly fields offer ways to enforce immutability, but they serve different purposes and have distinct behaviors. This article explores these two features, helping you decide when to use each based on your specific needs.

What is an Init Accessor?

Introduced in C# 9.0, the init accessor allows properties to be set during the object initialization phase—either via a constructor or through an object initializer. After initialization, properties with init accessors become immutable.

Example of an Init Accessor

public class Person
{
    public string Name { get; init; }
}

var person = new Person { Name = "John Doe" };
// person.Name = "Jane Doe"; // Compile-time error: Init-only property can only be set in an initializer

What is a Readonly Field?

Readonly fields in C# can be initialized either at the point of declaration or within the constructor of the class where they are declared. Once initialized, readonly fields cannot be changed.

Example of a Readonly Field

public class Person
{
    public readonly string Name;

    public Person(string name)
    {
        Name = name;
    }
}

var person = new Person("John Doe");
// person.Name = "Jane Doe"; // Compile-time error: Readonly field cannot be assigned to (except in a constructor)

Key Differences Between Init and Readonly

Mutability Context:

Init Accessors: Allow properties to be mutable during object initialization but immutable thereafter.
Readonly Fields: Must be initialized upon object creation and remain immutable afterward. They are usually set only in constructors.

Use Case:

Init Accessors: Ideal for object-oriented scenarios where properties need to be set during initialization and must remain unchanged afterward. They enable property setting via object initializers and constructors alike.
Readonly Fields: Best suited for more traditional class designs where immutability is enforced directly in constructors without the syntactic sugar of object initializers.

Flexibility:

Init Accessors: Provide more flexibility than readonly fields by allowing properties to be set in more places (constructors, property initializers, and with object initializers).
Readonly Fields: Are more restrictive, limiting setting to constructors, which can enhance data encapsulation and integrity.

Encapsulation:

Init Accessors: Encapsulate data but allow it to be easily displayed and used in data-binding scenarios, common in frameworks like ASP.NET or when using Model-View-ViewModel (MVVM) patterns.
Readonly Fields: Often used when there is no need for data-binding or when the data does not need to be exposed as a property.

When to Use Init Accessors vs. Readonly Fields

Use init when:

You need the data to be mutable during initialization using various syntactic methods but immutable afterward.
The class interacts with technologies that require properties, such as serialization frameworks and ORMs.

Use readonly when:

You want to ensure that data is only set during the construction phase and nowhere else, providing a strong guarantee of immutability.
The field does not need to be exposed as a property, or you are working within a context that doesn't benefit from the additional features provided by properties.

Conclusion

Choosing between init accessors and readonly fields in C# largely depends on your specific requirements for immutability, initialization flexibility, and the architectural design of your application. By understanding the strengths and limitations of each, you can effectively decide which feature to use to ensure your data remains consistent and your applications are robust and maintainable.

c# init vs readonly